The present invention relates to a rubber composition for coating a steel cord and a tire using the rubber composition.
Generally, since a heavy load is applied to tires for an automobile, a steel cord is used as a reinforcing material. Particularly during running, if a rubber and a steel cord are peeled off due to heat generation of tires, crucial breakdown is caused on the tires.
Conventionally, carbon black is mainly used as a reinforcing filler for a rubber composition for coating a steel cord. However, in recent years, interests in environmental conservation on the earth has been increasing and automobiles are not exceptional, thus, request for improving the fuel efficiency at a low cost of tires becomes strict, and further, since petroleum resources are limited, the use of materials derived from the petroleum resources such as carbon black also has been realized to be limited, therefore, the rubber composition compounded with a material such as silica is studied. However, when silica is compounded, processability and adhesivity are deteriorated.
As a means for improving processability, a process of using specific silica is known. However, there is a problem of being inferior in reinforcing property.
As a means for improving adhesivity, a process of compounding a large amount of sulfur is known. However, free sulfur without bonding with a rubber and a steel cord becomes large in a rubber composition after vulcanization, the sulfur is degraded with heat and oxygen during using tires, and rubber strength (stress at break and elongation at break) is lowered, which causes a problem of generating coagulation failure inside of the rubber composition in a duration test.
Further, in general, a steel cord is coated with a brass-plated layer, and cupper ion and zinc ion in the brass-plated layer are reacted with sulfur in a rubber to form an adhesive layer. When the adhesive layer is formed, if the reaction proceeds too fast, a sufficiently rigid adhesive layer cannot be formed, and the adhesive condition between the rubber and the steel cord is deteriorated. On the other hand, the reaction speed is too slow, an adhesive layer is hardly formed, and the adhesive condition between the rubber and the steel cord is also deteriorated in this case as well. Therefore, in order to sufficiently improve adhesivity, it is necessary that not only sulfur is compounded in a large amount, but also this reaction speed is adjusted.
JP-A-2003-64222 discloses an ecological tire excellent in durability by having a breaker topping comprising NR, silica, a silane coupling agent, zinc oxide and sulfur, but processability and adhesive condition cannot be improved.